1. Field of the Invention
The present invention relates to an object-detecting system for a vehicle for detecting an object such as a vehicle traveling ahead of a subject vehicle (hereinafter referred to as “a preceding vehicle”) by a radar system using FM-CW wave (frequency modulation continuous wave).
2. Description of the Related Art
A conventional object-detecting system for a vehicle is known from Japanese Patent No. 3305624.
As shown in FIG. 10, in the conventional object-detecting system using the FM-CW wave, a transmitting operation of a generator 3 is controlled in a modulated manner by an FM modulation control circuit based on a timing signal input thereto from a timing signal generating circuit 1, and a transmitted wave having a frequency modulated into a triangular waveform as shown by a solid line in FIG. 11 is transmitted from a transmitting/receiving antenna 6 through an amplifier 4 and a circulator 5. When a reflection wave resulting from the deflection of this FM-CW wave from an object such as a preceding vehicle is received by the transmitting/receiving antenna 6, the received wave appears behind the transmission wave at a frequency lower than that of the transmitted wave on an ascending side where the frequency of the transmitted wave is increased rectilinearly, and the received wave appears behind the transmitted wave at a frequency higher than that of the transmitted wave in a descending side where the frequency of the transmitted wave is decreased rectilinearly, as shown by a broken line in FIG. 11 in accordance with a distance to the object.
The wave received by the transmitting/receiving antenna 6 is input to a mixer 7 through the circulator 5. In addition to the wave transmitted from the circulator 5, a transmitted wave dispensed from the transmitted wave output from the generator 3 is input to the mixer 7 through the amplifier 8, and the transmitted wave and received wave are mixed together in the mixer 7, thereby producing a beat signal having a peak frequency Fup on the ascending side where the frequency of the transmitted wave is increased rectilinearly and having a peak frequency Fdn on the descending side where the frequency of the transmitted wave is decreased rectilinearly, as shown in FIG. 11.
The beat signal produced in the mixer 7 is amplified to an amplitude of a necessary level by the amplifier 9 and subjected to an A/D conversion at every sampling time by an A/D converter 10, and the digitized amplified data are stored and retained with time in a memory 11. A timing signal is input to the memory 11 from the timing signal generating circuit 1, and the memory 11 stores and retains data on every ascending side where the frequency of each of the transmitted and received waves are increased and every descending side where the frequency of each of the transmitted and received waves are decreased.
The data stored and retained in the memory 11 are input to CPU 12 including a frequency analyzing means 13, a detection peak determining means 14 and an object detecting means 15, and calculation based on the input data is carried out in the CPU 12.
The frequency analyzing means 13 is adapted to carry out frequency analysis of the data of the beat signal stored in the memory 11 to provide a spectral distribution. FFT (fast Fourier transform) is used as a technique for the frequency analysis.
The detection peak determining means 14 is adapted to detect a spectrum (a peak signal) in which the detection level is a maximum value at a predetermined detection threshold value or more, based on spectral data provided by the frequency analysis in the frequency analyzing means 13. When the relative speed of the subject vehicle and the preceding vehicle is zero, as shown in FIG. 11 an ascending-side peak signal and a descending-side peak signal overlap each other, but for example, when the subject vehicle is traveling at a relative speed to come close to a stationary object, as shown in FIG. 12, an ascending-side peak signal and a descending-side peak signal are detected symmetrically on opposite sides of a peak position at the time when the speed of the subject vehicle relative to the object is zero.
The object detecting means 15 is adapted to calculate a distance from the subject vehicle to an object and a speed of the subject vehicle relative to the object based on the ascending-side peak frequency Fup and the descending-side peak frequency Fdn provided by the detection peak determining means 14.
When an FM modulation width is represented by Δf; a light speed is represented by c; a modulation-repeating cycle is represented by Tm; a distance between the subject vehicle and an object is represented by r; a transmitted wave center frequency is represented by f0; and a relative speed between the subject vehicle and the object is represented by v, the ascending-side peak frequency Fup is given according toFup=(4·Δf·r)/(c·Tm)+{(2·f0)/c}·v  (1)and the descending-side peak frequency Fdn is given according toFdn=(4·Δf·r)/(c·Tm)−{(2·f0)/c}·v  (2)Here, when the FM modulation width Δf, the modulation-repeating cycle Tm and the transmitted wave center frequency f0 are supposed to be constant, the ascending-side peak frequency Fup and the descending-side peak frequency Fdn are represented by the following equation using constants k1 and k2, respectively:Fup=r·k1+v·k2  (3)Fdn=r·k1−v·k2  (4)
As apparent from the equations (3) and (4), when no relative speed v exists between the subject vehicle and the object (v=0), the ascending-side peak frequency Fup and the descending-side peak frequency Fdn are equal to each other (see FIG. 11). When a relative speed v exists between the subject vehicle and the object (v≠0), the ascending-side peak frequency Fup and the descending-side peak frequency Fdn are not equal to each other (see FIG. 12). A distance r to the object can be calculated based on the sum of the peak frequencies Fup and Fdn, and the relative speed v of the subject vehicle relative to the object can be calculated based on a difference between the peak frequencies Fup and Fdn.
When there are two preceding vehicles traveling at different relative speeds ahead of the subject vehicle, an ascending-side peak signal and a descending-side peak signal are generated for each of the preceding vehicles and hence, a total of four peak signals are provided. Therefore, if the pairing of the four peak signals (the combination of the ascending-side peak signals and the descending-side peak signals forming pairs) is erroneously performed, the following problem is encountered: distances from the subject vehicle to the objects and speeds of the subject vehicle relative to the objects cannot be detected accurately.
The object-detecting system described in the above-described patent document is designed so that the pairing is carried out based on past hysteresis data processed with time. However, the pairing is carried out for the combination of all the peak signals and for this reason, a long time is required for determining results of the pairing, leading to a problem in applying the object-detecting system to a system requiring a rapid control such as a vehicle-vehicle distance control system.
A particular problem in the case where the vehicle-vehicle distance control or the like is carried out using a radar device, lies in distinction between data for a moving object such as a preceding vehicle and data for a road-side object such as a guardrail and a sidewall. Therefore, if the moving object and the road-side object can be distinguished easily and reliably from each other, the performance of the radar device can be enhanced remarkably.